Metal halide lamps have been known as metal vapor discharge lamps to which reasonable mercury lamp ballasts are applicable. In general, a quartz arc tube body mainly is used in the metal vapor discharge lamps. However, in recent years, a ceramic arc tube body also is used to increase the heat resistance of the metal vapor discharge lamps.
FIGS. 33A and 33B are cross-sectional views, each showing one example of a conventional arc tube body formed of a ceramic material. FIG. 33A shows a conventional arc tube body including a cylindrical main tube portion 101, thin tube portions 102a and 102b for accommodating a pair of main electrodes, and ring-shaped members 103 for fixing the thin tube portions 102a and 102b to the main tube portion 101 (see JP 11(1999)-162416 A). On the other hand, FIG. 33B shows a conventional arc tube body including a thin tube portion 102c for accommodating an auxiliary electrode in addition to the same components as those in the arc tube body shown in FIG. 33A (see JP 10(1998)-106491 A).
In the arc tube body shown in FIG. 33A, the main tube portion 101 is formed by rubber pressing. On the other hand, in the arc tube body shown in FIG. 33B, the main tube portion 101 is formed by performing extrusion molding and then blow molding. In the arc tube body shown in FIGS. 33A and 33B, the thin tube portions 102a, 102b, and 102c are formed by extrusion molding, and the ring-shaped members 103 are formed by die pressing. The components formed independently as described above are connected with each other and then subjected to firing to complete an arc tube body.
However, the arc tube body shown in FIGS. 33A and 33B has the problems as follows. In the arc tube body shown in FIGS. 33A and 33B, the components are formed independently as described above. Therefore, when the arc tube body is used as an arc tube body of a metal vapor discharge tube, internal stress generated due to an increase in the internal pressure at the time of electric discharge is concentrated at the connecting portions between the respective components. In particular, regions 104, which are within the connecting portions between the main tube portion 101 and the ring-shaped members 103 and in the vicinity of the inner walls of the main tube portion 101, have low mechanical strength. Thus, cracks may be generated in the reigns 104 due to the internal stress.
In addition, in the case where components used for manufacturing an arc tube body are formed independently as described above, the process for connecting the components is required, which increases the cost for manufacturing the arc tube body.
As a solution to the above-mentioned problems, a slip casting method is proposed in which an arc tube body is formed integrally (see JP 11(1999)-204086 A). FIG. 34 is a cross-sectional view of an arc tube body formed by the conventional slip casting method. In FIG. 34, reference numeral 100a denotes thin tube portions for accommodating electrodes, and reference numeral 100b denotes a main tube portion to serve as a discharge space.
FIGS. 35 to 38 are cross-sectional views, each illustrating one process of the conventional slip casting method. It is to be noted that the processes illustrated from FIG. 35 through FIG. 38 are a series of processes. Hereinafter, a method for manufacturing an arc tube body according to the conventional slip casting method will be described with reference to FIGS. 35 to 38.
First, as shown in FIG. 35, a slurry 111 containing ceramic powder, a binder, and water as main components is injected to fill a hollow space inside a plaster mold 110. The hollow space inside the plaster mold 110 is formed so as to correspond to the external shape of an arc tube body to be manufactured.
Next, as shown in FIG. 36, only water from among the above-mentioned three main components contained in the slurry 111 is absorbed in the plaster mold 110, and a mixture 112 of the ceramic powder and the binder are allowed to adhere to the inner surface of the plaster mold 110 until it forms a sufficient thickness to provide a molded article with a desired thickness.
Subsequently, as shown in FIG. 37, excess slurry present in the hollow space is drained and the mixture 112 adhered to the inner surface of the plaster mold 110 is dried. Thereafter, a molded article 113 is taken out of the plaster mold 110. The molded article 113 is then subjected to an after processing such as firing. Thus, an arc tube body as shown in FIG. 34 can be obtained.
However, the slip casting method illustrated by FIGS. 35 to 38 has the following problem. When forming a small arc tube body of a low wattage, e.g., 70 W or less, thin tube portions 100a (see FIG. 34) are formed to be very thin. Thus, the thin tube portions 100a may be broken when being taken out from the plaster mold 110 or during transport.
Further, in the slip casting method illustrated by FIGS. 35 to 38, the arc tube body is formed by having water absorbed in the plaster mold 110, thereby adhering the mixture of the ceramic powder and the binder to the surface of the plaster mold 110. Therefore, from a macroscopic viewpoint, it can be said that this method can produce an arc tube body with a uniform thickness only. On this account, it is difficult to make only the thickness of tapered portions at the boundaries between the respective thin tube portions 100 and the main tube portion 100b greater than the thickness of other portions, for example.
Even in the case where an arc tube body is formed by the above-mentioned slip casting method, the thickness of the arc tube body can be changed partially by mechanically processing the molded article, for example. However, such mechanical processing increases the cost for manufacturing the arc tube body.
Further, a luminescent lamp provided with an arc tube body manufactured according to the slip casting method illustrated by FIGS. 35 to 38 may fail to light up. The reason for this is considered that calcium contained in the plaster mold 110 as a main component may adhere to the surface of the hollow molded article 113, which is to be processed into an arc tube body.
Therefore, it is an object of the present invention to solve the above-mentioned problems and to provide a method for manufacturing an arc tube body, capable of forming an arc tube body integrally and of reducing the chances that thin tube portions of the arc tube body might be broken, and a core used in the method.